Straight or shaped?
If you will use your skins for touring and weight and glide are of primary concern, a straight skin may be your best choice. If you climb a lot of vertical and your skis are considered "super-sidecut" (greater than 25mm difference between tip and waist), trimming your skins to fit the shape of your skis will give you more climbing performance.
For straight skins:
Subtract 4-6 mm from ski waist width. For example, if your ski waist is 74mm, a 70mm skin will fit properly.
For trim-to-shape skins:
Subtract 6-10 mm from ski tip width. For example, if your ski tip width is 108, choose a 100mm skin.